Details:

This has been fixed by the Xorg developers in commit
6c15122163a2d2615db7e998e8d436815a08dec6
(https://cgit.freedesktop.org/xorg/xserver/commit/?h=server-1.19-branch&id=6c15122163a2d2615db7e998e8d436815a08dec6).
This has been fixed in Cucumber Linux by the patch
http://mirror.cucumberlinux.com/cucumber/cucumber-1.0/source/x-base/xorg-server/patches/CVE-2017-12178_6c15122163a2d2615db7e998e8d436815a08dec6.patch